The present invention generally relates to electrochemical cells having conductive cathode additives and, more particularly, to alkaline electrochemical cells having cathodes formed of a mixture of manganese dioxide and a conductive carbon material for forming a conductive matrix.
Typical alkaline electrochemical cells generally include a steel cylindrical can having a cathode containing manganese dioxide as the active material and formed on the interior surface of the steel can, an anode having zinc powder and binding agent dispensed in the central portion of the cell, a separator located between the anode and the cathode, and an alkaline electrolyte contacting the anode, cathode, and separator. A conductive anode current collector is generally inserted in the anode active material, and a cover and seal assembly closes the open end of the steel can.
In conventional alkaline cells, the cathode is generally configured as the positive electrode, while the anode is configured as the negative electrode. The cathode typically comprises a mixture of electrolytic manganese dioxide particles as the cathode active material, graphite as the electroconductive material, potassium hydroxide solution, and deionized water. The conductive graphite is added to enhance the electrical conductivity between the manganese dioxide particles as well as to enhance electrical conductivity between the steel can and the cathode active material, since the manganese dioxide has an extremely low specific conductivity. As a consequence, the conductive graphite provides an electroconductive matrix amongst the active manganese dioxide particles as well as with the steel can.
A primary goal in designing alkaline cells is to increase the service performance of the cell. The service performance is the length of time for the cell to discharge under a given load to a specific voltage at which the cell is no longer useful for its intended purpose. One approach taken to increase service performance has been to increase the internal volume of the cell in order to increase the amount of active materials that may be employed within the cell. However, the commercial external size of the cell is generally fixed, thereby limiting the ability to increase the amount of active materials within a particular size cell.
Within the cell, a higher content of graphite mixed with cathode active material may enhance the electrically conductive properties of the cathode. However, a larger amount of graphite also consumes a larger volume, which, in turn, causes a reduction in the amount of the manganese dioxide active material that can be used in the cathode. This can lead to a reduction in the discharge capacity of the battery. Contrarily, a decrease in the amount of graphite may allow for an increase in the amount of active manganese dioxide that can be used, but this may lead to an increase in the internal resistance of the cathode which may decrease the cell""s discharge performance. Accordingly, it is desired to achieve a suitable compromise between the proportion of conductive material and manganese dioxide active material contained within the cathode of the cell.
Various attempts have been made for improving the discharge characteristics of electrochemical cells, particularly at high discharge rates. Since the conductivity of manganese dioxide is relatively low, the conductivity is ordinarily conferred on it by mixing in auxiliary conductive agents, such as graphite and/or acetylene black. However, the graphite and acetylene black, which are commonly used as the conductive agents, have intrinsic drawbacks, which include large volume consumption and high absorption characteristics.
In addition, conventional conductive additives employed in the cathode of the cell have included high levels of impurities which leads to an increase in gassing. Excessive gassing of hydrogen is known to decrease the performance of the cell. Accordingly, it is desirable to minimize the amount of gassing in the cell. Despite past increases in service performance with such additives, the need to find new ways to increase service performance remains the primary goal of the cell designers.
The present invention improves the service performance of alkaline electrochemical cells by the addition of an enhanced conductive graphite material to the cell""s cathode. To achieve this and other advantages, and in accordance with the purpose of the invention, as embodied and described herein, the cathode of the electrochemical cell according to one aspect of the present invention includes an active cathode material, such as electrolytic manganese dioxide, combined with an electrically conductive carbon, comprising expanded graphite particles, for providing a conductive matrix which occupies low volume. The expanded graphite particles preferably have a kerosene absorption in the range of 2.2 to 3.5 ml/g. The cathode of the present invention is particularly adapted for use in electrochemical cells having an alkaline electrolyte.
In accordance with another aspect of the present invention, the expanded graphite particles have a purity level of greater than 99.9%.
These and other features, objects, and benefits of the invention will be recognized by those who practice the invention and by those skilled in the art, from reading the following specification and claims, together with reference to the accompanying drawings.